Gas diffusion electrodes are well known and consist of a hydrophobic substrate layer, usually of PTFE, on one surface of which there is deposited a catalytic layer of controlled wettability. The side of the hydrophobic substrate containing the catalyst is placed in contact with an aqueous electrolyte; the other side is in contact with a gas. Gas, e.g. oxygen, can diffuse through the porous PTFE sheet and come into a contact with the catalyst wetted with electrolyte, where it can react electrochemically. Current from the reaction can be drawn from the electrode using a current collector.
Gas diffusion electrodes are made by several routes:    1) A slurry containing catalyst powder and small PTFE particles is dispensed into a well formed in a sacrificial medium, e.g. a foil. The use of a well allows the slurry to contain a high proportion of suspending liquid, e.g. water, which improves the accuracy of the amount of catalyst dispensed and also reduces the tendency of the catalyst to coagulate. A surfactant may be included in the slurry to reduce the tendency of the catalyst to coagulate. The dispensed slurry in the cavity is then dried by driving off the water (or other low boiling point carrier solvent) via low temperature oven cycles. The dried electrode is then cured at high temperatures to decompose organic stabilisers and thickeners, e.g. surfactants and polymers. If heated too quickly, the surfactants can combust, causing a sharp increase in temperature, which damages the electrode and can render it useless. Accordingly, tightly controlled heating rates are critical to the production of consistent electrodes. Once all the organic stabilisers have been decomposed, the catalyst is transferred to a PTFE tape by laying the tape across the top of the cavity containing the catalyst and the tape and the foil carrier are lightly pressed together between two flat surfaces and then transported to a press where they are subjected to a large pressure for a controlled amount of time. The foil is then peeled away, leaving the catalyst on the PTFE.            A disadvantage of the above process is that the controlled temperature regime required makes this process long drawn out and therefore expensive. Also, the transfer of the catalyst from the foil to the PTFE tape is often imperfect with some of the catalyst sticking to the foil. Additionally, the foil material can poison the catalyst, possibly alloying with the catalyst metal or leaving metal residues which may affect the catalyst activity.            2) Gas diffusion electrodes can be produced by screen printing a catalyst-containing ink onto a PTFE sheet. The rheology of the ink is critical. Unfortunately, because the ink is exposed to air during the screen printing process, it can dry out, which changes the rheology of the ink. Furthermore, the ink will contain substantial quantities of organic surfactants which must be decomposed by complex thermal treatment. If the temperature of the thermal treatment is too high, the surfactants will combust and the heat given out by the combustion will damage the electrode by localised thermal modification of the porous PTFE substrate. If the organic surfactants are left in, this will compromise the performance of the electrode by reducing the hydrophobicity of the substrate.            After screen printing, the deposit is pressed to form the final electrode.            3) The catalyst can be spray deposited onto a substrate through a mask. The substrate can be a sacrificial substrate. The required catalyst area is punched out from the sprayed substrate and the catalyst is transferred to a PTFE substrate using pressure as described in method 1) above. The disadvantages of this method are similar to those described above in connection with method 1).    4) The catalyst can be directly sprayed onto a sacrificial carrier, using a mask to create the desired electrode shape, and the catalyst deposit subsequently transferred to the PTFE substrate. This process has the same disadvantage and advantages as catalyst transfer mentioned in method 1) above and many of the disadvantages of screen printing in method 2); it has the additional drawback of providing lower catalyst utilisation because a high percentage of the catalyst-containing liquid is deposited onto the mask.
It will be appreciated that the accurate deposition of a precise dose of catalyst is essential to the manufacture of these gas electrodes since the catalyst is generally expensive (e.g. platinum) and since the amount of catalyst affects the performance of the electrode in that a catalyst loading that varies from electrode to electrode can give rise to different outputs, which is problematic when the electrodes are used for quantitative measurement, e.g. in gas sensors.
The present invention provides a process that provides consistent electrodes with low wastage of expensive catalyst and that is readily subject to mechanisation.